1. Field of Invention
The present invention relates to the field of mobile communications technology, and more particularly to a robust downlink timing synchronization method in an LTE system.
2. Description of Related Arts
In an LTE system, a base station of a cell sends a primary synchronization signal (PSS) carrying a cell identifier every 5 ms, and one of the main functions of the primary synchronization signal is downlink synchronization.
The PSS of the LTE system is generated by ZC sequences. There are three groups of different sequences in use. During the cell search process, the group of PSS sent by the base station is blind for the user. Therefore, when PSS is used for timing synchronization, the user generally stores three groups of local PSS sequences in advance, and each group of PSS has sliding correlation with a received signal respectively, and when an obvious correlation peak is detected, it is determined that the timing information is obtained. Such processing has a problem that the correlation operation of three groups is complicated which is disadvantageous for rapid acquisition of the timing information. In addition, if the user is located in a synchronous network, and performs correlation by using local synchronization sequences and received sequences, synchronization signals sent by other cells will have interference with the received signal of the current cell. Therefore, the method has different detection performance for different frequency offsets. In particular, when the frequency offset is large, the detection performance is poor. In addition, although this method has good timing performance within high signal-to-noise ratio intervals due to fine correlation properties of the PSS sequences. However, when the signal reception energy is low, that is, when the signal-to-noise ratio is low, the method has poor performance.
In the initial synchronization stage, the receiver has not obtained the signal offset in the received signal and the channel state of the user. Therefore, to ensure good timing synchronization performance, a timing method must have the following features: (1) being insensitive to the frequency offset, and (2) being capable of working normally when the signal-to-noise ratio is low. In other words, in practical applications, the timing method must have good robustness to the frequency offset and signal-to-noise ratio.